Known processes for applying corrosion resistant protective coatings on aluminum substrates are typically sequential in nature and utilize a chemical or electro-chemical surface treatment followed by the application of an organic primer. The aluminum substrate or component is first anodized, then sealed through hydration and subsequently the sealed anodic coating is coated, typically with an organic primer.
The multiple separate and distinct process steps of the above-noted coating operation result in a build-up of layers which creates dimensional problems due to the film thicknesses of the anodic coating and the primer. This build-up of layers results in an ultimate coating having a durability which is critically dependent on the degree of chemical/mechanical bonding between layers. Moreover, the entire operation for producing the multi-layered coating requires an appreciable amount of time and labor.
In an attempt to overcome the shortcomings of the known coating processes, the inventors of the present invention disclosed in Bradley et al. (4,310,390), assigned to the assignee of the present application, a protective coating process which reduces the number of process steps required to form a protective coating. The reduction of process steps is achieved by introducing a water-borne, water soluble acrylic resin into the sealing step of an otherwise conventional anodizing sequence to thereby simultaneously seal and impregnate the anodic coating. Subsequent to the sealing step, however, the resultant coating is cured at elevated temperatures up to 500.degree. F. The requirement of a heating step is not only costly in terms of production time and energy, but requires the maintenance of a precisely controlled temperature/time range which is difficult to achieve.
An improved coating composition and a simplified process to produce a protective coating for providing corrosion protection for metals is also disclosed by U.S. Pat. No. 4,515,919 to Bradley et al., assigned to the assignee of the present application, wherein an anodic coating applied to aluminum and alloys thereof is simultaneously sealed and impregnated at temperatures in excess of about 170.degree. F., in a time/temperature relationship, the coating being cured without the need for elevated cure temperatures. Although the coating is operationally quite efficient, the protective coating process of the '919 patent requires complex and expensive facilization in order to prevent accelerated solution aging and polymerization skinning of the resin when scaled up from a pilot line configuration to a production scale.